I can think of several ways to cause an object to rotate around its pitch axis.

I believe that the pitch axis normally corresponds to a mesh's x axis in Celestia if the bow of its mesh points in Celestia's +z direction. It's the same axis that an object turns around when one specifies its Inclination in an SSC file.

One should be able to do it directly using quaternions either in a ScriptedRotation or in a pre-defined SampledOrientation.

I'd probably use UniformRotation to generate the appropriate spin of an object in Celestia.

UniformRotation causes an object (or a coordinate system) to spin around its z axis in Celestia.

One technique to make it usable would be to use a 3D design program to change the orientation of the mesh so that its pitch axis corresponds to Celestia's z axis.

A second technique might be to use the SSC Orientation directive to change the default orientation of the object.

A third technique would be to define another SSC object which is at the same position as the object to be rotated, and specify the orientation of the object to be rotated so that it is at 90 degrees with respect to that first object. Then rotate that first object.